FIG. 1 shows a starter motor D which includes an electric motor M connected between ground and a power supply terminal B+ at the battery voltage.
A contactor 1 connected between said terminal B+ and the electric motor M controls the supply of power to the motor.
The contactor 1 is a relay with a moving core (not shown) actuated by an actuator coil 2 and a latching coil 3 respectively for pushing a power contact 1a into a closed position and for holding it there.
The actuator coil 2 is connected between the coil 3 and the side of the motor M that is not connected to ground. The opposite end of said coil 3 is connected to ground.
At their common end, the coils 2 and 3 are connected to the source of a transistor 4, the drain of which is connected to the terminal B+ via the starter switch 6.
A microprocessor 5 also connected to the power supply terminal B+ applies a control voltage to the gate of the transistor 4 to control the transistor 4 on an on/off basis, for example. As shown in FIG. 1, for example, the microprocessor 5 is integrated with the relay 1 and the transistor 4 in the starter motor casing. It can equally well be external of the starter motor, anywhere on the vehicle.
When said transistor 4 turns on, both the actuator coil 2 and the latching coil 3 are energized simultaneously.
To obtain a high actuation force, the actuator coil 2 has a much lower resistance than the latching coil 3. Since the resistance of the electric motor M when stationary is negligible compared with the resistance of the coils 2 and 3, the current flowing through said transistor 4 is at a maximum as long as the contactor 1 is not closed. This causes intense and fast heating of the transistor 4.
The forces generated by the coils 2 and 3 of the contactor move the core which closes the power contact 1a at the end of its travel.
The point 7 between the coil 2, the motor M and the contact 1a is then at the B+ potential. The coil 2 then draws virtually no current since both its ends are very close to the supply voltage at the terminal B+.
The transistor 4 then energizes only the latching coil 3, which draws little current, so heating of said transistor 4 is considerably reduced.
However, the power contact 1a may be prevented from closing properly, for example because of particles of insulative material on the faces of the contact 1a or because of mechanical jamming of components of the relay. The coil 2 is then energized continuously via the transistor 4 because its end connected to the motor M (point 7) remains at a potential close to ground potential.
The transistor 4 is then very quickly destroyed by overheating.